Studies of the Inclusion Complexation between a 3H-Indole and β-Cyclodextrin in the Presence of Urea, Sodium Dodecyl Sulfate, and 1-Propanol

The interaction of 2-( p -(methylamino)phenyl)-3,3-dimethyl-5-carboethoxy-3 H -indole ( 4 ) with β-cyclodextrin (β-CD) in the presence of urea, sodium dodecyl sulfate (SDS), and 1-propanol, respectively, were studied by absorption and steady-state fluorescence measurements. It was found that the addition of urea gives rise to a remarkable decrease in K 1, the association constant for the 1:1 (guest:host) inclusion complex, and a much more decrease in K 2, the association constant for the 1:2 inclusion complex. The linear correlations between ln K 2 and molecular volumes of some 3 H -indoles in the absence and presence of urea, respectively, indicate the hydrophobic nature of the interaction of 4 with either β-CD or urea. This result also shows that the urea effect study is an effective way to characterize the hydrophobic nature of the interactions between cyclodextrins (CDs) and guest molecules. Using 4 as a fluorescent probe, the interaction of SDS with β-CD was investigated through a competitive method. The formation of 1:1 and 1:2 inclusion complexes between SDS and β-CD was confirmed. The association constants K 1 ‘ for the 1:1 complex and K 2 ‘ for the 1:2 complex were estimated to be 26800 ± 2500 and 440 ± 90 M -1, respectively. Effect of 1-propanol on the inclusion complexation between 4 and β-CD was also investigated. The mechanism that 1:1:1 but not 1:2:2 ternary complexes of 4, 1-propanol, and β-CD are formed was demonstrated to be operative by a nonlinear regression routine. On the addition of either urea, SDS, or 1-propanol into the 4 -β-CD binary system, the fluorescence quantum yields (Φ F ) are significantly reduced suggesting that 4 transfers from the relatively nonpolar environments to polar environments.